This is incorrect. Packages currently in the distribution can and do
"Suggest" library packages required for some of their applications to
run, and I have yet to see anything in the Debian policy documents that
expressly forbids this practice.

What happens if I try to build pcmcia-cs on a system without libforms
and libforms-dev? Does it build cleanly or does it crash and burn?
"source-depends" on non-free packages also force a move to contrib.